"I suspect this is the case for making aliyah, but under what other circumstances would the validity of one's conversion be called into question?"
1) If she converts Conservative, then only Conservative and Reform Jews will recognize her or her children as Jews (I could talk about why, but that's best left for another question). If she makes aliyah (which is possible, I believe, even with a Conservative conversion), her children will not be able to get married in Israel unless they convert Orthodox, because marriage can only occur through the (Orthodox) rabbinate.
2) If she or her children or other descendents ever decide they want to become Orthodox (a fairly frequent occurrence for Conservative Jews, and bound to become more common as the share of the Jewish population that is Orthodox continues to rise), then they will have to convert.
3) If she or her children visit or spend time in an area where the only Jewish presence is Orthodox (as with much of Israel, and many out of the way places with Chabad houses), or foreign countries (such as in Europe) where Reform or Conservative never caught on, then they will not be able to fully participate in Jewish life. They could attend services or events, I suppose, but would not be able to be counted in a minyan, receive an aliyah, or officially join the shul.
"What is the benefit of converting under one denomination or another?"
1) Converting Conservative is a fairly long process, which often takes about a year and involves learning Hebrew, attending weekly services, observing kashrut and shabbat to Conservative standards (the official standards, not the norms that most Conservative Jews actually practice). In practice, she has to be much more observant than the rest of the congregation, perhaps even the rabbi. She has to confirm that this is a lifelong commitment that she will not neglect or cast off if she divorces or never marries him.
2) Converting Orthodox can take about the same amount of time, and involves the same requirements, but with some additional ones (expectations for kashrut, shabbat and taharat mispachah are somewhat more strict, and she will probably be expected to cover her hair after getting married.) The advantage is that virtually everyone in the whole world will accept her, her children and all her descendants as Jewish. She will also be less likely to be expected to be more observant than the other people in her congregation or community (except potentially in the case of a congregation where women don't cover her hair, as discussed in another Mi Yodea question).
3) To convert Orthodox, her born-Jewish spouse-to-be will need to become observant enough for her to convert (especially in terms of kashrut and shabbat), and he has to be willing to follow the rules of taharat mishpacha. He will need to be willing to join an Orthodox shul. This could be a benefit or disadvantage, depending on your perspective. (See the book DoubleLife by Harold and Gayle Berman for one very interesting and moving account of a family becoming Orthodox together.)
4) An advantage of converting Orthodox (even though it is somewhat more difficult), is that Orthodox Judaism "works." Most Orthodox Jews grow up, stay observant, get married to other Jews, and have Jewish children. The sad truth is that non-Orthodox Jews have extremely high intermarriage rates, and are much less likely to get married at all (and when they do, it tends to be in their 30s). See this summary of a recent population study (see websites such as simpletoremember.com as well).
5) Another advantage of converting Orthodox is that Orthodoxy is a "big tent," with a wide range of viewpoints and practices. The non-Orthodox tend to think of the Orthodox as fanatical or right-wing, but in truth there is a huge amount of diversity, and there are many liberal and lenient Orthodox Jews. Two exit polls even found that a majority of Orthodox Jews voted for Obama in 2012. Orthodox Jews, both men and women, are found in virtually every profession.